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Hawaiian Islands Synoptic Discussion and Guidance

506
FXHW60 PHFO 261335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Fri Feb 26 2021

.SYNOPSIS...
Strong and gusty trade winds will persist across the region into
early next week due to a series of strong surface highs to the
north and northeast of the islands. Rainfall will remain focused
over windward slopes, but an upper level disturbance will give an
added boost to these trade showers. The disturbance may even kick
off a few thunderstorms Friday and through the weekend over the
eastern end of the island chain.

&&

.DISCUSSION...
The early Friday morning surface map shows a very strong and
broad 1047 mb surface high sitting 1600 miles north-northeast of
the islands. It is the main source of the strong and gusty
easterly trade winds blowing across the main Hawaiian Islands.

A Wind Advisory is in effect for the rest of today for parts of
the Big Island and Maui County. An extension of this advisory is
possible, through tonight. We will observe the trend before making
this decision later today. But the trades will be softening
slightly as this high begins moving toward the southeast and
weakening some. There were a couple wind alarms overnight, namely
Kalaupapa on Molokai and the Kohala Ranch on the lee side of the
Kohala Mountains Big Island.

Satellite imagery shows a field of widely dispersed showers mostly
upwind of Maui and the windward coast of the Big Island from
Upolu Point to the Puna district. Despite the limited cloud cover
on satellite, the Koolau Mountains on Oahu have been quite wet in
the past couple of hours.

As noted above, the trade winds will be easing off slightly
tonight as the high shifts southeastward. A new high, although not
as strong, 1043 mb, is forecast to moving into range by Sunday
giving the trade winds another boost. The high then drifts south
and gradually weaken through the first half of next week. Under
this scenario, trade winds are likely to regain very wind status
by Saturday night, possibly leading to another Wind Advisory for
Maui County and northern Big Island. It won't be until Wednesday
where the trade wind will down to the normal speed of moderate to
locally strong.

An upper level low with a pool of cold air of minus 13 to minus
14 degree C at 18k feet will be drifting over the islands from
the north in the next 12 to 18 hour, with most of the dynamics
situated over the Big Island. This destabilization of the air mass
over the islands will enhance the trade wind showers. making some
of the showers briefly heavy. Thanks to the breezy trades, these
showers will be moving along as passing showers. The winds will
also carry the showers well into the lee areas of the smaller islands
before dissipating.

The upper low will also be drawing some mid moisture from the
south and over the Big Island on Saturday, leading to some frozen
precip for the Big Island summits. Further more, there is a slight
chance of a thunderstorm, especially on Saturday for the Big Island
and east Maui. The upper low over the islands weakens into a trough
Saturday night, and warms slightly aloft. Models are pointing to
a slightly drier Sunday for the smaller islands, while the Big Island
remains showery, along with a slight threat of a thunderstorm.
The ECMWF and GFS shows the upper trough moving just south of the
Big Island Monday, thus ending the threat of spotty heavy showers
and thunderstorms for the Big Island, while a more typical windy
trade wind pattern continues elsewhere. As noted above, the trade
winds should be back to its normal moderate to locally strong by
Wednesday.

&&

.AVIATION...
Strong easterly trade winds will continue today, sustained by an
area of impressive high pressure currently located north-
northeast of the main Hawaiian Islands. AIRMET Tango remains in
effect to account for low level mechanical turbulence for areas
over and immediately south through west of mountains.
Additionally, since strong winds are forecast to persist
throughout the day, AIRMET Tango also accounts for surface winds
greater than 30kts in the Alenuihaha Channel and south of the Big
Island. Low level wind shear will also continue to be a concern at
a few airports.

As an upper level low moves into the area today, an uptick in
shower activity is expected, particularly along eastern sections
of the islands. MVFR level clouds and reduced visibility in
heavier showers will be possible for eastern portions of the Big
Island this morning, followed by isolated thunderstorms on
leeward slopes of the Big Island this afternoon. Otherwise, VFR
conditions will prevail throughout the period.

&&

.MARINE...
A very strong high northeast of the Hawaiian islands and a trough west
of the area are creating a tight pressure gradient that is producing
strong east winds. A Gale Warning is in effect for the Alenuihaha
Channel and the Big Island Leeward Waters through this evening. A
Small Craft Advisory (SCA) is in effect for the rest of the
Hawaiian Coastal Waters through the weekend. Winds will subside a
bit tonight as the high weakens. Winds are likely to strengthen
again over the weekend as a new high build north of the area.
Winds could reach gale force again Sunday night or Monday.

A long fetch of strong northeast and east trade winds is producing
elevated seas. A High Surf Advisory in effect for exposed east
facing shores will remain up for the next several days. Surf
elsewhere will be below advisory levels. Several long-period
northwest swells are expected over the next week. These swells
will peak today, tomorrow night into Sunday, and Wednesday into
Thursday. The mid-week swell is expected to be the largest of the
three. Some trade wind swell will wrap around into north and
south facing shores. A small swell from the southern hemisphere
will also continue for the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST this evening for Molokai-Kahoolawe-
Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Big Island Leeward Waters.

Gale Warning until 6 PM HST this evening for Alenuihaha Channel-
Big Island Leeward Waters.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Southeast
Waters.

&&

$$

H Lau/Vaughan/Kinel/Donaldson

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

Current Conditions
Temp		-4.8 C
RH		97 %
Wind		ENE 14 mph
Road		Closed